With the development of high-resolution optoelectronic imaging devices and high-performance computers in the last two decades, traditional microscopic measurement mainly based on optomechanics has developed into microscopic vision measurement characterized by automatic imaging in high resolution and rapid processing in real time. Microscopic vision measurement technology, with its rapid, accurate and automatic imaging and analysis, has been widely applied in geometric metrology of tiny biological organs and small industrial workpieces, and also applied in visual guidance in cell injection and Micro Electro Mechanical System (MEMS) microoperation.
Precision of microscopic vision measurement, which is highly demanded for the development and manufacture of micro elaborate assembly, is the main concerned problem in actual applications. It is difficult to perform microscopic vision measurement calibration due to the narrow depth of field in microscopic optics, which has become a hot research issue at home and abroad. For example, Danuser (“Photogrammetric calibration of a stereo light microscope”, Journal of Microscopy, Vol. 193, 6283, 1999) proposed a measurement model for a stereo light microscope, assuming some of model parameters are true (for example, the principal point and the distance 2b between baselines of left and right parallel light paths). Bundle adjustment method was applied for calibration based on 3D target constructed by a high-precision mobile platform, and a measurement precision of 0.25% laterally and 0.60% axially in the magnification of 6.6× can be achieved in the distance measurement. However, the precision of the above microscopic vision measurement is apparently inferior to that of conventional vision measurement which could be higher than 0.05‰. For example, Wu et al (“A Novel Calibration Method for Large-Scale Stereo Vision Sensor based on One-dimensional Target”, 7th International Symposium on Instrumentation and Control Technology, Proceedings of SPIE, Vol. 7129, 2008: 71290K) developed a large-scale binocular vision measurement system based on unconstraint 1D target, which could even achieve a precision of 0.04‰ in distance measurement.
Compared with the precision of conventional vision measurement, the precision of microscopic vision measurement is still inferior, which is mainly caused by its low calibration precision. Due to the strong nonlinearity of the optimization objective function in calibration, optimization is sensitive to initial values, therefore when initial values are ill-defined, which is the exact case in microscopic vision measurement where closed-form solutions are inaccurately derived in the narrow depth of field with the image plane approximately parallel to the target plane, it is difficult for a function with the closed-form solution as an initial value to converge to the global optimal value, sometimes the parameter to be calibrated is even an imaginary point.